1. Field of the Invention
This invention relates to an optical element and an optical system having the same, and particularly to an optical element provided, for example, with incidence and emergence surfaces and a plurality of reflecting surfaces on the surface of a transparent member, and an optical system having the same. This optical element and this optical system are suitable for image pickup apparatuses, such as a video camera, a still video camera and a copying apparatus for forming an object image on the surface of an image pickup element.
2. Related Background Art
There have heretofore been proposed various photo-taking optical systems utilizing a reflecting surface such as a concave mirror or a convex mirror. FIG. 11 of the accompanying drawings is a schematic view of a so-called mirror optical system comprising a concave mirror and a convex mirror.
In the mirror optical system of FIG. 11, an object light beam 104 from an object is reflected by a concave mirror 101, travels toward the object side while being converged, and is reflected by a convex mirror 102, whereafter it is imaged on an image plane 103.
This mirror optical system uses the construction of a so-called Cassegrainian reflecting telescope as its basis, but shortens the full length of the optical system by folding the optical path of a telephoto lens system of a long full lens length comprised of a refracting lens, by the use of two reflecting mirrors opposed to each other.
Also, in an objective lens system constituting a telescope, for a similar reason, a number of types shorten the full length of the optical system by the use of a plurality of reflecting mirrors.
Thus, by using reflecting mirrors instead of a photo-taking lens of a long full lens length, the optical path has heretofore been efficiently folded to thereby obtain a compact mirror optical system.
Generally, however, in a mirror optical system such as a Cassegrainian reflector, there is the problem that part of an object ray of light is eclipsed by the convex mirror 102. This problem is attributable to the fact that the convex mirror 102 is in the passage area of the object light beam 104.
In order to solve this problem, there has also been proposed a mirror photo-taking optical system which uses an eccentric reflecting mirror to prevent another portion of the optical system from shielding the passage area of the object light beam 104, i.e., to separate the principal ray of the light beam from an optical axis 105.
FIG. 12 of the accompanying drawings is a schematic view of a mirror photo-taking optical system disclosed in U.S. Pat. No. 3,674,334, and this optical system solves the above-mentioned eclipse problem by making the center axis itself of a reflecting mirror eccentric relative to the optical axis to thereby separate the principal ray of the object light beam from the optical axis.
The mirror optical system of FIG. 12 has a concave mirror 111, a convex mirror 113 and a concave mirror 112 in the order of passage of the light beam, and each of them originally is a reflecting mirror, rotation-symmetrical with respect to an optical axis 114, as indicated by dots-and-dash lines. Use is made of only the upper portion of the concave mirror 111 with respect to the optical axis 114, only the lower portion of the convex mirror 113 with respect to the optical axis 114, and only the lower portion of the concave mirror 112 with respect to the optical axis, as viewed in the plane of the drawing sheet of FIG. 12, whereby there is constructed an optical system in which the principal ray 116 of an object light beam 115 is separated from the optical axis 114 and the eclipse of the object light beam 115 is eliminated.
FIG. 13 of the accompanying drawings is a schematic view of a mirror optical system disclosed in U.S. Pat. No. 5,063,586. The mirror optical system of FIG. 13 solves the above-noted problem by making the center axis itself of a reflecting mirror eccentric relative to an optical axis to thereby separate the principal ray of an object light beam from the optical axis.
In FIG. 13, when the vertical axis of an object surface 121 is defined as an optical axis 127, the central coordinates and center axes of the reflecting surfaces of a convex mirror 122, a concave mirror 123, a convex mirror 124, and a concave mirror 125 in the order of passage of the light beam (axes linking the centers of the reflecting surfaces and the centers of curvature of those surfaces together) 122a, 123a, 124a and 125a are eccentric relative to the optical axis 127. In FIG. 13, the amounts of eccentricity at this time and the radius of curvature of each surface are appropriately set to thereby prevent the eclipse of the object light beam 128 by each reflecting mirror and to cause an object image to be efficiently formed on an imaging plane 126.
Besides, U.S. Pat. No. 4,737,021 and U.S. Pat. No. 4,265,510 also disclose a construction using a portion of a reflecting mirror rotation-symmetrical with respect to an optical axis to avoid eclipse, or a construction in which the center axis itself of a reflecting mirror is made eccentric relative to an optical axis to thereby avoid eclipse.
FIG. 14 of the accompanying drawings shows an afocal optical system for observation using four reflecting surfaces, which is disclosed in U.S. Pat. No. 5,309,276. In FIG. 14, third and fourth mirrors 203 and 204 are disposed so that a light beam from an object may be reflected by a first mirror 201, a second mirror 202 and the third mirror 203 and may pass the front of the first mirror 201 twice, and then may emerge perpendicular to the incident light and may be imaged on a pupil 205. An observer's pupil is situated at 205.
As optical elements in which a number of reflecting surfaces are made into a block, there have heretofore been developed optical prisms, such as a pentagonal roof prism which are and a porro prism used, for example, in a finder system or the like.
In these prisms, a plurality of reflecting surfaces are formed as a unit and therefore, the relative positional relation between the reflecting surfaces is made with good high accuracy, and positional adjustment between the reflecting surfaces becomes unnecessary. However, the main function of these prisms is to change the direction of travel of a ray of light to thereby effect the reversal of an image, and each reflecting surface is constituted by a flat surface.
In contrast with this, there is also known a photo-taking optical system in which a curvature (refractive power) is given to the reflecting surface of a prism.
FIG. 15 of the accompanying drawings is a schematic view of the essential portions of an observation optical system disclosed in U.S. Pat. No. 4,775,217. This observation optical system observes a scene in the external world and also observes a display image displayed on an information display member in overlapping relationship with the scene.
In this observation optical system, a display light beam 145 emerging from the display image on an information display member 141 enters from the incidence surface 148 of a prism member, is reflected by a surface 142, travels toward the object side and enters a concave surface 143 comprising a half mirror. It is then reflected by this concave surface 143, whereafter the display light beam 145 is made into a substantially parallel light beam by the refractive power of the concave surface 143, and is refracted by and transmitted through the surface 142, whereafter it enters an observer's pupil 144 and makes the observer recognize the enlarged virtual image of the display image.
On the other hand, an object light beam 146 from the object enters a surface 147 substantially parallel to the reflecting surface 142, is refracted thereby and arrives at a concave surface 143 comprising a half mirror. Half-transmitting film is deposited by evaporation on the concave surface 143, and a part of the object light beam 146 is transmitted through the concave surface 143 and is refracted by and transmitted through the surface 142, whereafter it enters the observer's pupil 144. Thereby, the observer visually confirms the display image in overlapping relationship with the scene in the external world.
FIG. 16 of the accompanying drawings is a schematic view of the essential portions of an observation optical system disclosed in Japanese Laid-Open Patent Application No. 2-297516. This observation optical system also observes a scene in the external world and observes a display image displayed on an information display member in overlapping relationship with the scene.
In this observation optical system, a display light beam 154 emerging from an information display member 150 is transmitted through a flat surface 157 constituting a prism Pa, enters the prism Pa and impinges on a reflecting surface 151 comprising a parabolic surface. The display light beam 154 is reflected by this reflecting surface 151 and becomes a convergent light beam, and is imaged on a focal plane 156. The display light beam 154 reflected by the reflecting surface at this time arrives at the focal plane 156 while being totally reflected between two parallel flat surfaces 157 and 158 constituting the prism Pa, whereby the thinning of the entire optical system is achieved.
Next, the display light beam 154 having emerged as divergent light from the focal plane 156 enters a half mirror 152 comprising a parabolic surface while being totally reflected between the flat surface 157 and the flat surface 158, and is reflected by this half mirror surface 152 and at the same time, forms the enlarged virtual image of the display image by the refractive power thereof and becomes a substantially parallel light beam, and is transmitted through the surface 157 and enters an observer's pupil 153 to thereby make the observer recognize the display image.
On the other hand, an object light beam 155 from the external world is transmitted through a surface 158b constituting a prism Pb, is transmitted through the half mirror 152 comprising a parabolic surface, is transmitted through the surface 157 and enters the observer's pupil 153. The observer visually confirms the display image in overlapping relationship with the scene in the external world.
Further, as examples using an optical element as the reflecting surface of a prism, there are optical heads for optical pickup disclosed, for example, in Japanese Laid-Open Patent Application No. 5-12704, Japanese Laid-Open Patent Application No. 6-139612, etc. These are such that light from a semiconductor laser is reflected by a Fresnel surface or a hologram surface, whereafter it is imaged on the surface of a disc, and the reflected light from the disc is directed to a detector.
On the other hand, the assignee, as shown in FIG. 17 of the accompanying drawings, has proposed a mirror optical system whose downsizing is achieved by the use of an optical element in which a plurality of curved reflecting mirrors or flat reflecting surfaces are formed as a unit and yet in which the disposition accuracy (assembly accuracy) of the reflecting mirrors liable to be in the mirror optical system is made loose. In FIG. 17, reference numeral 51 designates an example of an optical element in which a plurality of curved reflecting surfaces having curvatures are formed as a unit, i.e., an optical element comprising, in succession from the object side, a concave refracting surface R2, five reflecting surfaces such as a concave mirror R3, a convex mirror R4, a concave mirror R5, a convex mirror R6 and a concave mirror R7, and a convex refracting surface R8, and the direction of a reference axis entering the optical element 51 and the direction of a reference axis emerging from the optical element 51 are substantially parallel and opposite to each other. Reference numeral 52 denotes an optical correcting plate, such as a rock crystal low-pass filter or an infrared cut filter, reference numeral 53 designates the surface of an image pickup element such as a CCD, reference numeral 54 denotes a stop disposed on the object side of the optical element 51, and reference numeral 55 designates the reference axis of a photo-taking optical system.
Describing the imaging relation in FIG. 17, light 56 from the object has its quantity of incident light regulated by the stop 54, and thereafter enters the concave refracting surface R2 of the optical element 51.
The light having entered the concave refracting surface R2 is reflected by the concave mirror R3 after the object light 56 is made into divergent light by the power of the concave refracting surface R2, and primarily forms an object image on an intermediate imaging plane N1 by the power of the concave mirror.
The object light 56 primarily imaged on the intermediate imaging plane N1 repeats reflection by the convex mirror R4, the concave mirror R5, the convex mirror R6 and the concave mirror R7 and comes to the convex refracting surface R8 while being affected by the power of each reflecting mirror, and the object light 56, refracted by the power of the convex refracting surface R8, forms an object image on the image pickup element surface 53.
As described above, the optical element 51 functions as a lens unit having a desired optical performance and positive power as a whole while repeating the refraction by the incidence and emergence surfaces and the reflection by the plurality of curved reflecting mirrors having curvatures.
A mirror optical system of this kind is also disclosed in Japanese Laid-Open Patent Application No. 9-258104.